Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical elements, such as lenses and mirrors etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image formed on a mask, reticle or the like onto a substrate such as a wafer. Said optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical element units. Such optical element units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually comprise an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element.
Optical element groups comprising at least mainly refractive optical elements, such as lenses, mostly have a straight common axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, the optical element units holding such optical element groups often have an elongated substantially tubular design due to which they are typically referred to as lens barrels.
Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture and increased imaging accuracy of the optical system.
Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the optical elements of such an optical system is desirably supported in a defined manner in order to maintain a predetermined spatial relationship between said optical elements to provide a high quality exposure process.
In this context there exist, among others, two general requirements for the support of optical elements of the optical system. One is that the rigidity of the support system of the optical elements has to be as high as possible in certain directions, in particular in the direction of the optical axis, to keep the resonant frequencies of the system as high as possible. Furthermore, deformations of the optical elements of the optical system are to be avoided to the greatest possible extent in order to keep imaging errors resulting from such deformations as low as possible.
One such imaging error is for example stress induced birefringence of refractive optical elements. Such stress induced birefringence mainly results from stresses introduced into the optical element via its peripheral support structure and radially propagating through the optically used area of the optical element. Such stresses are often thermally induced, resulting from differences in the coefficient of thermal expansion (CTE) of the optical element and its peripheral support structure. Variations in the temperature situation of the optical element and its peripheral support structure lead to relative movements between the optical element and its peripheral support structure. These relative movements are counteracted by the holding forces acting between the optical element and its peripheral support structure leading to the above undesired stress situations.
To avoid thermally induced stresses and deformations within an optical element due to differences in the coefficient of thermal expansion of the optical element and its optical element holder, it is known to connect the optical element and its optical element holder via deformation uncoupling elements. These deformation uncoupling elements generally allow for relative movements between the optical element and its optical element holder.
These deformation uncoupling elements may provide a reduction of the stresses and, thus, the deformations introduced into the optical element. However, they have the disadvantage that they also reduce the rigidity of the support system. To deal with this effect, the rigidity of the uncoupling elements might be increased, but this would reduce their deformation uncoupling abilities leading to increased stresses and, thus, the deformations introduced into the optical element.
Another approach to deal with this problem is known from US 2001/0039126 A1 (to Ebinuma et al.). Here, it is provided for an adaptation of the coefficients of thermal expansion between an optical element and a support ring contacting the optical element in order to reduce the introduction of thermally induced deformations into the optical element resulting from differences in the coefficients of thermal expansion. However, this solution my have the disadvantage that, for certain optical elements with a certain coefficient of thermal expansion, the adaptation of the coefficient of thermal expansion may only be achieved with comparatively expensive materials for such large parts as the support ring.